Aliphatic polyisocyanates have been widely used as curing agents for paints excellent in weather resistance. Particularly, a polyisocyanate which is produced from hexamethylene diisocyanate (hereinafter referred to as HDI) has been widely utilized for various applications such as building construction, automobiles and the like. However, such a polyisocyanate which is obtained by oligomerizing a diisocyanate, has a relatively high viscosity and in many cases needs to be diluted with various organic solvents before use. However, in the field of paints, reduction of the amount of organic solvents used is an urgent problem to be solved from the viewpoints of environmental protection against air pollution and resource saving. Under such a situation, a polyisocyanate composition, low in viscosity, has been demanded because of easy handling and in order to reduce the amount of organic solvents used.
As a polyisocyanate low in viscosity, there has been known a monoallophanate product formed from HDI and an organic compound having one hydroxyl group. However, the product has a problem in that it has a small number of functional groups, namely 2, and is inferior in curability.
JP-A-4-306218, JP-A-2-250872, JP-A-5-70444, and JP-A-5-222007 propose a process for obtaining a low viscosity polyisocyanate by reacting a diisocyanate and a monovalent alcohol to form a urethane and then trimerizing the urethane with an isocyanurate-forming catalyst. However, the product of this process consists mainly of an isocyanurate product, with which a monoallophanate product obtained from the monovalent alcohol is combined. By this process, it is difficult to design a polyfunctional polyisocyanate having more than three functional groups.
As a polyisocyanate composition readily soluble in an organic solvent of low polarity, JP-A-9-12660 proposes an isocyanurate type polyisocyanate having an allophanate group, which is obtained by using, as starting compounds, a monovalent alcohol having a carbon number of 6-20, and a diol having a carbon number of 4-40. However, the polyisocyanate disclosed in this publication also consists mainly of an isocyanurate product. Therefore, when it is attempted to design a polyfunctional polyisocyanate having more than three functional groups, the molecular weight becomes higher. Particularly, when a diol is present, the molecular weight becomes remarkably high by isocyanurate-forming reaction. Thus, it is difficult to obtain a low viscosity polyisocyanate composition having a high number of functional groups.
Polyisocyanates having an allophanate group are disclosed in British Patent 994, 890, JP-A-46-1671, JP-A-64-66155, JP-A-7-304724, JP-A-8-188566 and so on. Their specifications disclose many working examples but do not suggest actual compositions of a polyisocyanate having low viscosity, a high number of functional groups, and excellent curability.
JP-A-54-14921 discloses that allophanate-forming reactions are conducted by use of an acid catalyst. With regard to an alcoholic hydroxyl compound for producing a polyisocyanate having an allophanate group, from monovalent to tetravalent alcohols are shown therein as the examples thereof. It is shown therein that the functionality of the resulting polyisocyanate having an allophanate group can be controlled in a wide range by adequately selecting the hydroxyl compound. However, there is not disclosed a polyisocyanate composition having low viscosity, a high number of functional groups, and excellent curability. Furthermore, the process using a strong acid disclosed in this publication has a problem in that allophanate-forming ability is not so high. Therefore conducting the reaction for a long time at a relatively high temperature is necessary and therefore the resultant polyisocyanate may be discolored in some cases.
JP-A-64-66155 discloses a process for producing a polyisocyanate having an allophanate group by heating for a short time at a high temperature. There are shown the working examples wherein hexamethylene diisocyanate (HDI) and monovalent and divalent alcohols are used as starting compounds, but there is not disclosed a polyisocyanate composition having a low viscosity and a high number of functional groups. In the process disclosed therein, since allophanate-formation is conducted by heating, it is considered that a molecular weight-increasing reaction such as isocyanurate-forming reaction occurs. Therefore, in the product obtained in the working examples, the content of isocyanate group is lower than the value expected from the composition, and the viscosity disclosed is higher than the expected value.
As a low viscosity polyisocyanate, JP-A-9-216930 discloses a mixture of a polyisocyanate having a viscosity of more than 700 mPa·s and an aliphatic (alicyclic) triisocyanate having a viscosity of less than 200 mPa·s. A polyisocyanate formed by using the aliphatic (alicyclic) triisocyanate tends to form a harder and brittler coating film, as compared with general polyisocyanates formed by using HDI and the like, and is of limited availability and thus difficult to employ for industrial production at present. In addition, this publication discloses as a comparative example a mixture of a monoallophanate product obtained from a monovalent alcohol and an isocyanurate product, which has low molecular weight and insufficient strength and furthermore low number of functional groups. This publication discloses that a polyisocyanate having an allophanate group produced from an alcohol having a higher number of functional groups does not show an advantageous viscosity as compared with a polyisocyanate having a biuret group or an isocyanurate group, and shows that it is difficult to produce a low viscosity polyisocyanate by using a polyisocyanate having an allophanate group derived from a polyvalent alcohol.